Confining and enhancing light within deep subwavelength volumes is key to the enhancement of light-matter interaction, with great implications in the control of absorption and emission rates, as well as in attaining high optical nonlinearities and/or gain. However, certain applications also crucially require efficient power funneling of the confined and enhanced electromagnetic fields. Such a phenomenon was demonstrated for the first time in a thin silver film patterned with subwavelength-sized cylindrical holes. See T. W. Ebbesen et al., Nature (London) 391, 667 (1998). In that work, the transmitted light is beyond the expectations of Bethe's theory and twice the amount predicted from a simple analysis based on the area fraction of the holes. See H. A. Bethe, Phys. Rev. 66, 163 (1944); and C. J. Bouwkamp, Rep. Prog. Phys. 17, 35 (1954). Therefore, such a phenomenon was termed as an extraordinary optical transmission (EOT). An important goal has been towards optimization of the pertinent features of the phenomenon: field confinement and enhancement, and power throughput exceeding the area fraction of the holes. See C. Genet and E. Ebbesen, Nature (London) 445, 39 (2007) and references therein. It is now widely accepted that the EOT phenomenon is a resonant phenomenon mediated by surface plasmon excitation. See H. F. Ghaemi et al., Phys. Rev. B 58, 6779 (1998); and F. J. Garcia-Vidal et al., Rev. Mod. Phys. 82, 729 (2010).
Resonant phenomena unavoidably have a narrow spectral bandwidth. There can be some rather limited control of the bandwidth by engineering the interaction between resonances. For example, by exploiting coupled resonances, a broadened bandwidth was observed in the enhancement around nanoparticle dimers or in the transmission through a metallic grating. Typically, this type of approach would require fine tuning of the structural features: size, shape, and/or angle of wave incidence. See S. Foteinopoulou et al., Opt. Express 15, 4253 (2007); X. R. Huang et al., Phys. Rev. Lett. 105, 243901 (2010); X. Shi et al., Opt. Lett. 28, 1320 (2003); J. A. Matteo et al., Appl. Phys. Lett. 85, 648 (2004); K. Tanaka and M. Tanaka, Opt. Commun. 233, 231 (2004); and L. Tang et al., Opt. Lett. 31, 1519 (2006). It is therefore of utmost interest to explore the possibility to access the attractive features of the EOT phenomenon, pertinent to practical applications, but without invoking any resonances. Very recently, Alu et al. reported a nonresonant approach to a broadband transmission of P-polarized light through a metallic grating structure, occurring at the Brewster angle of the corresponding effective medium. Nevertheless, such a platform does not offer a two-dimensional confinement of the optical fields and leads to poor field enhancement due to the reduced tangential electric field component. See A. Alu et al., Phys. Rev. Lett. 106, 123902 (2011). Moreover, the required oblique incidence at large angles can be considerably less practical.
Therefore, a need remains for a two-dimensional structure that simultaneously enables both E-field enhancement with subwavelength power throughput and broadband transmission at near-normal incidence.